A high-efficiency furnace typically employs several heat exchangers to warm an air stream passing through the furnace. The heat exchanger may include “clamshell” or individual panel halves formed by stamping mirror images of the combustion and exhaust chambers into corresponding metal sheets. The halves are then typically fastened together with what the industry calls “button mushrooms,” which are often located on an outer perimeter of the metal sheets of the clamshell heat exchanger or similar fastening configurations.